[Observability] Add production-safe sampling and OTLP export primitives

[cssbruno]

Problem

GOWDK has useful tracing primitives, but production export/sampling controls are still too thin for a safe default toolbox.

Verified in this checkout:

• runtime/trace has AlwaysOn, AlwaysOff, and deterministic RatioSampler.
• The sampling context includes trace ID and parent span ID, but there is no parent-based sampler helper or route/endpoint-specific override policy.
• The nested runtime/trace/otel bridge exposes endpoint, insecure mode, headers, batcher-backed export, and Shutdown.
• The bridge does not expose ForceFlush, queue/batch sizing, retry/backoff controls, compression, TLS options beyond insecure/default TLS, or resource attributes such as service name/version/environment.

Acceptance criteria

• Add parent-based deterministic ratio sampling primitives.
• Add generated/app-owned route or endpoint override hooks for health checks, noisy endpoints, and high-value endpoints.
• Expose bounded export queue and batch configuration, or document how apps supply an owned OpenTelemetry provider when they need that control.
• Define overflow/drop behavior and surface dropped/exporter-failure counters.
• Add bounded retry/backoff controls where GOWDK owns export, or explicitly delegate them to the app-owned provider path.
• Expose ForceFlush in addition to Shutdown.
• Support TLS/header/compression/resource attributes needed for common OTLP deployments.
• Document the split between GOWDK-owned stable primitives and app-owned sampling policy/telemetry backend selection.